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Author Topic: Problems threading on the lathe  (Read 330271 times)

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Offline RICH

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Re: Problems threading on the lathe
« Reply #450 on: September 28, 2009, 10:01:11 PM »
Hi All,
Took a look at the Turn Diagnostics for 402  and 115 rpm.
I just did a dry run of  the threading code at the 60 passes for scribing of the tube at both rpms.
There was a definite difference between the 115 and 432 rpm info:

During the higher rpm the Highest % variation during threading changed four times up to a high ending value of 18.4% while the low rpm only varied by up to an ending value of 1.37%. Additionaly,  the Mega Clock variation was greater for the higher rpm. Rotation real time rpm increased by 5 rpm for the higher and only 0.5 for the lower rpm. So there is a difference on what's happening with index pulse and also how the threading cycle is applying correction based on that info.

Now what is also interesting is that the % variation during threading was somewhat stepped for the 402 rpm, such that the first step change went from 10 to 18% after 15 passes, and gradualy creeped to the final 18.4% , which would kind of confirm what was observed when doing the actual scribing,.... at some point during the threading cycle there is a definite change in the Z axis movement.

So as posted before, that the 3/4-13 actualy cut at 115 rpm had no lead problems while the scribed one at the 402 rpm did,  this may just provide the explaination as to why.

The difference of when the five scribed lines "ended" is about  10 degrees from each other.
If i did the math correctly, that's o.oo3" of Z travel and that is quite close to what you can measure in the picture.

So i guess it's time to try a scribed line at the lower rpm and see if what i am thinking is just rubish
or not. Or i'm reading to much and incorrectly into the diagnostics info.

RICH

Offline ART

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Re: Problems threading on the lathe
« Reply #451 on: September 28, 2009, 10:20:08 PM »
Rich:

 I suspect your on to it. The problem in my mind may be that if a sensor varies.. its variance is pretty much set, lets say its a uSecond early on one pass, maybe 1us late on another.If the rpm is lower, that constant variance will be less as a % of the total time used. Your example was a 4 time differential between the two periods.. At 115rpm the 1us variance may be 5% of the total, but at 405 it may now reflect 20% of the total timing.. accuracy would fall as the constant lagging or leading variance would slowly make it go out of known position.

   Now it may not be the sensor.. it may be the printer port interrupting a bit late at times.. same effect..

I do know that if I feed a signal in my printer port from a frequency generator, its dead on and very repeatable, but when my spindle sensor is measured, its floating about 3us either way at times..
(I think its 3us.. its been a few weeks.)

Art
Re: Problems threading on the lathe
« Reply #452 on: September 28, 2009, 10:34:31 PM »
RICH,
 Just a thought. Do you think that a photo sensor with a Schmidtt Trigger would be more stable than a magnet and Hall sensor ?

Just wondering,
Russ

Offline RICH

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Re: Problems threading on the lathe
« Reply #453 on: September 29, 2009, 12:07:05 PM »
Russ,
I looked at the sensor signal and it's has a nice crisp on-off / high low of same duration and no ringing to it. My o'scope is not the best in the world and it does seem like the signal is floating some, it should be stable such that it triggers and appears on the scope in the exact same spot, but again i don't trust my scope.

The Schmidtt trigger is good because you can refine / deal with the hysteresis ( however you spell that). So i won't discount using it.

Art,
I tend to agree with you and it may be a combination of both the sensor a little out of time and the computer. There are four of us here that have the same problem. Now it's difficult to put it all into perspective since every system has their owne quirks which could play into the threading problem
differently.So i will stay with my system since the mechanical end is known not to be an issue.
I think i can get my friend  to lend me his computer to try as it has a very stable pps in the driver test.

One common problem observed is that somewhere during  the passes the Z makes a deeper cut.
Say at pass 10 out of 30 passes. I am thinking that it's when the % variance is accounted for.
I don't have a signal generator that i can use for a pulse. The one i borrowed had the range but can't vary the width of the pulse.

Would it be of any value to add to the Turn diagnostics the pass at which the change in % variance occurs or is implemented?

I will post shortly with a picture and more info since i have tried another scribing cut.

Thanks for the commnets,
RICH

Offline ART

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Re: Problems threading on the lathe
« Reply #454 on: September 29, 2009, 01:23:04 PM »
Rich:

  Each pass is its own entity, there is no difference from pass to pass. Correction occurs only during a pass, all knowledge is lost after the pass,
so if it cuts deeper overall, then it has to be the X depth or its fooling you because its cutting inot a crest making you think its deeper than it should be.
But there is only correction during a pass, and it knows nothing of previous passes..

Art

Offline RICH

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Re: Problems threading on the lathe
« Reply #455 on: September 29, 2009, 01:33:58 PM »
Here's another scribing test.
Attached are two pictures of a threading scribe test which was done at 115rpm and 60
( 0.0001" deep passes ). I used a 30 degree angle so it was radialy cutting thus no Z offsets in the Gcode. Picture 1 shows finished scribing and there was a definite improvement. Picture 2 shows
the end of the passes and the total width of all the cuts was cut in half as compared to the same at
402 rpm and there are five distinct cuts.  

Now i watched each cut under the microscope since i can see the actual tip point and what material is being removed. For the first 20 passes it tracked the first pass. Then there was a slight shift at pass # 30 about 0.002" increasing the lead.  Then at pass #46 the cuts started oscillating, such the one would be on the first pass line, then maybe the middle of the width, and finaly further down the lead. Pass #53 opened up the width some 1 or 2 thou ( lead increase ) and then pass #54 opened the width up around 1 thou ( lead decrease ). Passes after that kind of tracked the middle of the of all the passes.

So it seems that the sensor is not constantly stable, maybe.......since that's what the o'scope's telling me, but i don't trust it. But then it could be the PC or combination of both.

If anything, i can say it dosen't take much to screw up a thread.

And for those following this, when you post and ask a threading question, and maybe you get so many different thoughts on the matter, some generalized comment, now you know there may not be a simple answer to a system of mechanical, electrical and software all trying to work in harmony!

RICH
« Last Edit: September 29, 2009, 04:26:44 PM by RICH »

Offline RICH

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Re: Problems threading on the lathe
« Reply #456 on: September 29, 2009, 02:14:34 PM »
Thanks Art,
Depth is not even a concern for me, it's the lead / longer threading, as shown by what i just posted.
The x axis is very accurate in it's moves and the Z is almost neglagible.

I am going to guess that i can cut threads out to about 3/4" to 1" all day long and they will be good.
Hope i don't seem anal about the threading. For what i do this lathe will be fantastic.
 
So over time i will try a different PC and have a friend look at the sensor triggering / timing since he has
the equipment to do it and is extremely knowledgable.

What fooled me was the distinct cuts, and you got to watch each and ever one of them to get the story as i just
posted.

By the way, a good test of the sensor triggering  would be to do multiple start threads.
Another would be to pick the thread up and recut it ( it's somewhat of a bitch to do in CNC). I posted something in members docs on how pick the thread up.

I guess the thought here is to eliminate the problem such that no correction will be required. Maybe wishfull thinking with the computer and sensor currently in use, but, wouldn't know unless some testing was done.

As always appreciate the comments,

RICH

Offline ART

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Re: Problems threading on the lathe
« Reply #457 on: September 29, 2009, 03:23:37 PM »
Rich:

  Excellent testing!!

    I agree with your comments. Though I suspect a nut would screw on even to the end of the thread.

 Interesting that it happens at certain passes, that seems to indicate the sensor blipped a bit late or early in the trigger.
After triggering the thread, if the sensor did blip late a bit it probably wouldnt matter as it self corects during the pass, so
I suspect what we're seeing is that the sensor sometimes delays a bit ( or the computer does somehow ), and the problem
occurs when iut does it during the initial trigger of the thread.

  I have to note that of all tests on threading performed to date, yours are the most scientific, and the best documented.

Interesting that 20 passes were good, then the problem occured and toggled about between various pass points. It does seem as if
our collective problem is the timing of the initial start at times, not so much the correction during the pass. Of course you likely have very little
slowdown if any so the nice passes reflect a steady output from initial start point.

Ill dwell on it a bit..

Art

Offline RICH

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Re: Problems threading on the lathe
« Reply #458 on: September 29, 2009, 04:47:37 PM »
Art,
If the test's and documentation provide info such that you can improve the threading or others have a way to analyze what is wrong with their system, then it was worth the time and effort.
RICH

Offline RICH

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Re: Problems threading on the lathe
« Reply #459 on: October 01, 2009, 11:53:01 PM »
Art,

Here is some more testing and a little more interesting information.

I was able to borrow a different computer. The driver test for that computer showed a dead
straight line, no blips at all,  and the PPS only varied by a count of 2. That computer was used on test
sample #3 & #4 shown in picture #1.The testing with that computer was with the PP and the threading diagnostics showed the following:
CPU Interupt varability: 0 or .01%
Highest Variation during threading: 0.51%
Rpm Variation: .01 to .07
So I consider this pc about as good as your going to get.

All the thread scribing for the 4 tests was done at 115 rpm, 60 / 0.0001” passes with 30 degree ( no Z axis changes in the Gcode ).

Picture comments:
Picture #1: #1 & #2 were done with my pc and #3 & #4 were done with the borrowed pc.
Picture #2: This picture shows the improvement in the threading with a more stable pc.
Picture #3: Enlarged picture of test #4
Picture #4: This shows the cutting mark both at the beginning and end of a 2” long thread
                   for test #4. Note that it also is representative of test #3. There is only one SINGLE
                   cut line and it is consistent from start to finish of the thread in width and depth  by actual
                   measurement. ( the magnification on the pictures were different ).

COMMENTS:
So the stable pc did improve the varying cutting position as you suspected ( single verses multiple cut
 lines  in #1 & #2). I watched every pass, and there was no oscillating back and forth between the passes for #2 & #3 as compared to #1 & #2 test with my pc. That tells me the sensor is doing it’s job of triggering consistently.

Now you would think life is good. NOT SO.

The lead is increasing on each of the threads and the increase is the same root to root of adjoining cuts.
TEST#    PC                             TOTAL LEAD ERROR       LEAD ERROR IN 1”
--------     ----                             -------------------------------    -------------------------
2             MINE                        0.018” / 2”                              0.009”
3             BORROWED            0.016” / 2”                              0.008”
4             BORROWED            0.013” / 1.8”                           0.007”

( btw, The max lead error of a ground tap is 0.0005” in 1”.  )

What is causes the increasing lead error?

It’s not the mechanics of the lathe. I re-checked  the triggering, and my Z axis moves. This was done as follows: I did individual G32 –Z2.0 F.050  moves 50 times and the axis stopped each time within the Z backlash. So, that tells me that the triggering is quite accurate since it if the triggering varied much the end stopping point of the axis would be off. The stopping point was confirmed using optical equipment.

Next, I put test piece #4 in the chuck,…. picked up the thread 10 times ….. a little differently than what I posted some time ago because I am using Hall sensor, did G32 –Z ( starting at -1 and increasing the move in .1 increments out to Z-2.0” ) F0.50. The sharp tool point was very close but not actually touching the test piece.  This allowed me to check them z axis movement against the scribed test piece along with a guess of where the trigger actually occurs. I could measure the difference between the test piece and the z axis move. And it consistently showed the Z axis move had no error in the distance and I could also check against the know lead error of the test  piece. The measured difference was very close each time.

So it seems like the threading cycle is introducing a lead error from what I can tell.

Would it be possible to have an option in the threading cycle to “turn off the compensation during the threading” or maybe be able to put a value into it to adjust a know lead error? Just a thought!

RICH